In the United States, heart disease is a major health problem. Of the 1.5 million people per year who suffer a myocardial infarction, about 680,000 survive that have ischemia (dead heart tissue) which is the basis for cardiac arrhythmias. Approximately 400,000 people a year die from the most serious types of cardiac arrhythmias.
Arrhythmias can be classified into three broad types. Bradycardia is an abnormally slow heart rhythm. This problem has been successfully treated for a number of years with implantable pacemakers which induce the heart to beat at a faster, normal rhythm. The remaining types of arrhythmias are more difficult to control.
Tachycardia is a rapid cardiac rhythm generally defined as a heart rate greater than 100 beats per minute. There are normal physiologic tachycardias due to exertion or emotion as well as abnormal nonphysiologic tachycardias in which a high rate results in loss of blood pressure. Sustained ventricular tachycardia can result in severe loss of blood pressure, loss of consciousness and can deteriorate into ventricular fibrillation which is fatal if not quickly interrupted.
Fibrillation, unlike tachycardia, is a disorganized cardiac rhythm wherein the heart quivers rather than beats. This quivering is a result of multiple waves of cardiac depolarization spreading and colliding throughout the ventricular tissue. Ventricular fibrillation results in a precipitous decrease in blood pressure followed quickly by brain damage and death.
Arrhythmias are treated using either medication, surgery or implantation of a medical device. Drug therapy is employed initially in the majority of cases and involves the use of various medications to prevent an arrhythmia from starting or being sustained. The main advantage of drug therapy is that no surgical intervention is required. The major drawback to the exclusive use of therapy is the lack of backup therapy to terminate a potentially lethal arrhythmia should the drug eventually fail to prevent the arrhythmia from recurring. Additionally, in attempting to achieve adequate tachycardia prevention, drug related side effects often preclude using an adequate dose of medication.
Surgery involves locating the cause of the arrhythmia and removing or isolating it from the healthy cardiac tissue. The advantage of surgical therapy is that the procedure is curative when successful. The disadvantage of surgical therapy is the morbidity and mortality associated with open heart surgery and the technical difficulty and high cost of the procedure. These factors have restricted the practice of antiarrhythmia surgery.
In 1980, the first implantable defibrillator was implanted in a human patient. Implantable defibrillators sense fibrillation and automatically deliver a high energy pulse. Subsequent studies have indicated that these devices are effective in preventing sudden death from fibrillation. Presently, no single implantable device has been developed to control all three types of arrhythmias.
The analysis of patients who have arrhythmias often requires invasive testing in an electrophysiology lab. This invasive testing is carried out in a variety of situations. For example, invasive testing is common during a selection process used to determine which patients might be candidates for implantable defibrillators. Invasive testing is also utilized in trying to assess and characterize tachycardia which is then treated with drugs. In any case, in the testing process, catheters are inserted into the heart and the patient's arrhythmia is provoked with programmed electrical stimulation. When the arrhythmia manifests itself, the physician attempts to terminate it with antitachycardia pacing. Antitachycardia pacing is described in the literature and consists of a series of low voltage pulses designed to reset the normal heartbeat. If pacing fails, the patient is either cardioverted with a substantially higher voltage shock or defibrillated with a very high voltage energy pulse.
Low energy cardioversion utilizes pulses with energy levels far greater than pacing pulses but lower than high energy defibrillation pulses. With energies of less than 5 joules, this mode of therapy is based on the theory of interrupting the arrhythmia by stimulating the tissue, rendering it nonexcitable. Low energy cardioversion has been clinically demonstrated as effective. Its drawbacks include patient discomfort and the fact that improperly timed pulses can accelerate tachycardias and occasionally induce fibrillation.
In contrast, high energy defibrillation uses pulses with energy levels tens of thousands of times greater than pacemaker pulses. High energy defibrillation is accomplished by stimulating a large portion of the ventricular tissues simultaneously and rendering it nonexcitable, thereby terminating the arrhythmia. If a patient is found suitable, an internal defibrillator can be implanted to control the arrhythmia. During the operation, the patient's defibrillation threshold must be determined. First, the patient is fibrillated using a programmable stimulator, then a special defibrillator is used to determine the energy required to defibrillate the patient.
The invention described herein facilitates the assessment of arrhythmias and defibrillation thresholds resulting in improved patient care and substantially decreased patient risk. In the prior art, there existed both programmable stimulators and cardioversion/defibrillator devices. The programmable stimulator includes a means to pace the patient's heart with critically timed stimuli to provoke the cardiac arrhythmia. These devices are then used to terminate the arrhythmia using antitachycardia pacing. If the pacing accelerates the arrhythmia or fails to terminate it, then a standby defibrillator device must be set up and the patient cardioverted or defibrillated. Frequently, the patient is cardioverted or defibrillated externally. More recently, internal catheters have been provided to deliver the defibrillation shock.
As noted above, an external cardioverter/defibrillator is used during implantable defibrillator procedures to assess the patient's cardioversion/defibrillation threshold. When the patient is fibrillated using a programmable stimulator, the unit must be disconnected before a test defibrillation pulse can be applied. Frequently, the test pulse fails to defibrillate the patient. Once the physician recognizes the failure to defibrillate, he must program a new, higher voltage rescue shock into the defibrillator. The unit must then recharge prior to delivery of the rescue shock. This procedure takes considerable time and there is ample opportunity for operator error. Any delay in defibrillating the patient is a serious health risk and improving the response time to the delivery of the rescue shock substantially reduces patient risk. Accordingly, it would be desirable to eliminate any unnecessary time between delivery of the test shock and rescue shock.
In the above described procedures, it is also clear that in the electrophysiology lab, it is frequently necessary to use both a programmable stimulator and a cardioverter/defibrillator. Present day equipment requires the operator to switch leads and move back and forth between two pieces of equipment. During this procedure, care must be taken to prevent any of the high voltage charge delivered by the defibrillator from reaching the output leads of the programmable stimulator to avoid damaging the latter. Accordingly, it would be desirable to provide a single combination test unit wherein leads would not have to be changed and automatic protection of programmable stimulator would be provided.
Another drawback of the defibrillation devices available in the prior art relates to the fact that little or no measurement and visual feedback is given to the surgeon regarding the defibrillation pulse. More specifically, the surgeon typically sets a pulse width and a voltage level for a test shock. If this test shock fails, the surgeon cannot be sure whether it was the result of shorted leads, an unexpectedly high resistance in the heart or whether the voltage was just too low to stop the defibrillation. There presently exists some low voltage pacing devices which have been designed to provide additional information to the surgeon regarding patient resistance and energy delivered. However, to date, no systems have been provided to calculate and display this information in a defibrillation setting. In a life-threatening situation, such as cardiac fibrillation, such information is extremely important and can aid the surgeon in assessing the type of rescue shock necessary to end the fibrillation.
The energy delivered to the heart of a patient is generally measured in joules. The energy level of the shock is analogous to a dosage in therapy. In prior art devices, as in the subject invention, the surgeon sets the defibrillation shock by adjusting a voltage level and the pulse width. However, in the prior art devices, no information is given to the physician as to the energy which will be received by the patient if a shock with those set parameters were delivered. Therefore, it would be desirable to provide a device which displays the estimated energy based on the set voltage level and pulse width.
Accordingly, it is an object of the subject invention to provide a new and improved apparatus for electrophysiology testing in patients suffering from severe ventricular arrhythmias.
It is another object of the subject invention to provide a new and improved apparatus which advantageously combines a programmable stimulator and an internal cardioversion/defibrillation device.
It is a further object of the subject invention to provide a combination stimulator/defibrillator apparatus with automatic circuit protection for the stimulator.
It is a still another object of the subject invention to provide a new and improved defibrillator which includes a pair of capacitor banks permitting the simultaneous storage of both a test shock and a rescue shock.
It is still a further object of the subject invention to provide a new and improved defibrillator apparatus which includes automatic recharge circuitry to reduce the time necessary to deliver a rescue shock during an emergency procedure.
It is still another object of the subject invention to provide a new and improved apparatus which allows multiple, independent entries of data which are stored for later recall during testing procedures.
It is still a further object of the subject invention to provide a new and improved defibrillator apparatus which will display measurement of resistance and energy delivered during a defibrillation shock.
It is still a another object of the subject invention to provide a new and improved defibrillator apparatus which will display the energy which is estimated to be delivered if a shock of a given voltage and pulse width is to be delivered.